Hemp (Cannabis sativa x indica) is grown both outdoors and in greenhouses. The environmental conditions of each system varies extensively, resulting in differing disease pressure. High humidity greenhouses and other closed environments are often ideal for fungal diseases such as powdery mildew. Outdoor-grown hemp is much less susceptible to the disease.

Powdery mildew diseases are caused by a group of fungi that develop fungal strands and chains of fungal spores on leaf surfaces and branch tips. Powdery mildew pathogens are host specific; each species of the fungus is specific to certain hosts. Thus, the species that infects Cannabis are restricted to hemp, hops, and a limited number of host plants. Likewise powdery mildew pathogens of surrounding plants (examples dogwood powdery mildew or rose powdery mildew) will not infect Cannabis spp. Golovinomyces sp. (synonym Erysiphe sp.) remains the most common of the powdery mildew pathogens that infect Cannabis spp.

PATHOGEN BIOLOGY

Understanding the basic biology of the causal powdery mildew fungus is important for management of disease. Environmental conditions, presence of fungi, and plant susceptibility are major factors in the disease cycle. Thus, understanding their relationship is vital to management of powdery mildew in hemp. (Refer to Hemp Disease 101: Back to Basics for more details).

Powdery mildew develops on leaf surfaces as the familiar white powdery growth. Masses of white mycelia (fungal strands) and chains of conidia (asexual spores that are produced in large numbers) make up this powdery growth. Infective conidia are carried by air currents to healthy plant material; fans, wind, and mechanical means can move spores. Under ideal conditions (moderate temperatures and high humidity), an infection site of just one square centimeter can produce an average of 300,000 spores each day. Within 5 to 10 days after infection, diseased plant tissue becomes a source for new spores, and the cycle continues. Under greenhouse conditions, this repeating cycle is continuous.

In outdoor plantings or open greenhouses, a second fungal stage may occur. During stressful events such as drought, stress, or plant senescence, powdery mildew fungi produce survival structures (chasmothecia) that allow for survival during adverse conditions. Chasmothecia encapsulate sexual fungal spores and are hard and darkened with melanin. They are tolerant of heat, drought, freezing temperatures, and fungicides, hence the term survival structure. This is the typical means for overwintering/overseasoning. Powdery mildew fungi overwinter as chasmothecia in fallen leaves and other plant debris on the ground. When favorable conditions resume, chasmothecia release spores and initial infections begin the disease cycle.

MANAGEMENT

Disease management begins with cultural practices. By manipulating one or more of the major disease factors - environment, pathogen, and host – the disease cycle is broken or disrupted. Elimination of any one these factors will provide control of powdery mildew. It is often impossible to eliminate disease factors completely, however, but growers can consider a variety of approaches to significantly reduce disease. Use of cultural practices requires consistent implementation, and disease management will continue to improve each season.

Consider each of the factors for disease development, and manipulate each for disease management:

Host: Some varieties/cultivars may exhibit more tolerance to infections than others. Ideally, selection of resistant cultivars would be the one-and-done approach to managing disease. However, only minor resistance or tolerance may be available. Utilize any available resistance, and combine this approach with other cultural practices.

The developmental stage of hemp plants affects susceptibility to fungal pathogens, including the powdery mildew fungi. Young, vegetative, succulent tissue is more susceptible to infection than mature or hardened tissue. Thus, fast-growing and actively growing plants are at a higher risk for disease. Reduction of fertilizer and water can slow plant growth and may provide some resistance.

Environment: Fungal pathogens require two environmental conditions in order to infect and cause disease – moderate temperatures and available moisture (free water or humidity >95%). Manipulation of these environmental factors can suppress the powdery mildew fungus and reduce infection and spread.

Moderate temperatures are required for infection and disease development. In fact, optimal temperatures for conidial production is 65 to 70 degrees F. At temperatures at or above 86 degrees, infection and spore production are reduced by half, and at temperatures of 55 to 60 degrees, disease development can be delayed by one to two weeks. Thus, temperature manipulation can help slow the powdery mildew disease cycle.

Moisture is also required for the pathogen to infect and develop. High humidity provides sufficient moisture, and greenhouse environments are thereby ideal sites for disease development. Efforts to reduce greenhouse humidity are critical in management of fungal diseases such as powdery mildew. Air exchange may be needed to lower greenhouse humidity (reference). Even in controlled environments, fungal spores can take advantage of damp tissue and microclimates (pockets of high humidity such as between plants and inside plant canopies) that provide conditions conducive for infection (Figure). Sufficient air movement such as fans help dry leaves and circulate air. Space plants to improve air circulation, thus, lowering humidity between plants and within canopies (Figure). Reduce amounts of irrigation water, when possible, and avoiding overhead watering. In outdoor settings, orient rows to take advantage of natural air movement. Avoid planting in shaded locations.

Pathogen: Disease-causing fungi can be reduced or eliminated by means of fungicide or by sanitation. Sanitation is the act of cleaning and/or maintaining a pathogen-free environment. Prune infected plants, quarantine infected or susceptible plants, and discard severely affected plants in efforts to limit or reduce amounts of fungi and their infective spores. Sweep or rake fallen leaves and other debris, as they can harbor infective propagules, as well. Clippings and debris should be hauled away from growing sites and burned or buried, as appropriate. (See Sanitation article)

Fungicides remain a contentious issue, especially for growers who strive to maintain organic or non-sprayed products. There are no commercial fungicides that are labeled for Cannabis. Unfortunately, there are many reports of illegal use of fungicides and other pesticides. Although there are no registered fungicides that include Cannabis on their labels, there are some broadly-labeled products that contain language that does not exclude them from use. These products are primarily botanicals or other biological products. Read product labels carefully and follow instructions before use; the label is the law. Efficacy of botanical products has proven poor to fair, at best. Many of them are not effective at all. Combinations of cultural practices (air movement, plant spacing, sanitation, etc.) and botanical products, however, can provide adequate control.

SUMMARY

Overall, hemp are low-maintenance, disease-free plants that require few inputs. When grown indoors, however, they are susceptible to a range of fungal diseases, such as powdery mildew. High humidity greenhouses provide optimum conditions for disease development. By understanding the biology of the pathogen, growers can utilize cultural practices and manipulate growing conditions to reduce fungal spread and disease development.